jlbell Posted October 9, 2011 Report Share Posted October 9, 2011 This may belong in the Class III section, but it is a technical question!I plan to build an AR .308 SBR (Happen to have the 20" barrel out of a DPMS Classic - 129 rnds. fired) and I want to toy around with a Semi-Auto .308 SBR. Barrel will be cut to 12", gas port re-drilled, and the gas block area forward turned down to .750", with muzzle rethreaded. My question is which gas system length would work best? Obviously I cant use rifle length and not sure about mid-length, maybe Carbine length (?). Rifle will be shot suppressed at all times and used with surplus plinking ammo. I have a new JP Enterprises adjustable gas block (.750") to use. Any suggestions?Thanks, Quote Link to comment Share on other sites More sharing options...
Jgun Posted October 10, 2011 Report Share Posted October 10, 2011 I have no first hand experience building SBR's, but since mid length is going to be around 9.5 to 10 inches, I think that to be sure that you'll have no reliability issues, I'd be inclined to go with a carbine length gas system. I realize that mounting a dedicated suppressor might be enough to create the pressure needed to make a mid length gas system work on a 12" SBR, but unless you know of someone successfully running that setup, your going to be the one to prove whether or not, it's a good configuration. Have you done a search on the silencer talk forum to see if there's an answer to your question over there? Quote Link to comment Share on other sites More sharing options...
Jgun Posted October 10, 2011 Report Share Posted October 10, 2011 To expand on my last reply, I have a Noveske 7.62 14.5" Afghan barreled carbine, and Noveske (who know what their doing with SBR's) use what they call a carbine length gas system for their 14.5" 7.62 Barrel , but the location is the actually the same as what is considered a midlength for a 5.56 barrel. Now on their 12.5" 7.62 Leonidas barrel they state that they use a proprietary gas block location, which leads me to believe that it must be shorter than what they call carbine for 7.62 but is actually mid length for 5.56. I understand that none of the guns mentioned, are fitted with suppressors but I have to believe that you'd do best with a 5.56 carbine gas location on your 12" 7.62 barrel, even if suppressed. Quote Link to comment Share on other sites More sharing options...
jlbell Posted October 10, 2011 Author Report Share Posted October 10, 2011 JgunI have not checked on Silencer Talk yet, but will. With limited knowledge on running the 12" 7.62 barrel, I was thinking Carbine length or a little shorter. I want to run a standard length, not a one of a kind set-up, so figured using an adjustable gas block would give me some room to adjust for reliability. Quote Link to comment Share on other sites More sharing options...
Jgun Posted October 10, 2011 Report Share Posted October 10, 2011 That's what I would do if it was me. It's a lot easier to use an adjustable gas block to limit gas, than it is to try to get a gun that is undergassed by design, to run properly, besides, even if you plan to mount your suppressor permanently, you still may need to adjust gas to suit different loads you might run. Quote Link to comment Share on other sites More sharing options...
jlbell Posted November 21, 2011 Author Report Share Posted November 21, 2011 Just an FYI:LR-308 Barrel was cut (12.5"), recrowned, threaded, contoured (.933") 1.875" in length for adjustable (.936") JP Ent. Gas Block then contoured (.857") fwd of gas block to muzzle. Gas Port drilled .090" for carbine length gas system. Rifle comes in at 9 lb. 10.4 oz. W/Armson optic & carry handle. Rifle runs fine with surplus 7.62 ammo, both suppressed and un-suppressed. All parts were new and bolt needs to be sprayed with lube (CLP, etc.) fairly often. Parts are not worn in, yet, so has to be lubed. May have oversized the gas port just a bit. After running the weapon, .086" was probable big enough. But I'm very pleased with outcome!!!!!!!!!!!!!! Quote Link to comment Share on other sites More sharing options...
Robocop1051 Posted November 21, 2011 Report Share Posted November 21, 2011 WTG! <thumbsup> Quote Link to comment Share on other sites More sharing options...
Triggs75 Posted November 24, 2011 Report Share Posted November 24, 2011 Pics please Quote Link to comment Share on other sites More sharing options...
jlbell Posted November 24, 2011 Author Report Share Posted November 24, 2011 Pics pleaseI'll try and figure out how to post pics after the holiday! Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 24, 2011 Report Share Posted November 24, 2011 jlbell, You left out the key piece of information. We all like to share our technical information here. You never stated which length gas system you went with, 5.56 carbine length? You mention that you think the gas port might be a little large, does that mean that you've had to close the gas block adjustment a lot in order to get it to cycle? Can you give details on which can your using, for those of us that can run them? And finally, How quiet is it, do you still need ear pro? Quote Link to comment Share on other sites More sharing options...
jlbell Posted November 25, 2011 Author Report Share Posted November 25, 2011 jlbell, You left out the key piece of information. We all like to share our technical information here. You never stated which length gas system you went with, 5.56 carbine length? You mention that you think the gas port might be a little large, does that mean that you've had to close the gas block adjustment a lot in order to get it to cycle? Can you give details on which can your using, for those of us that can run them? And finally, How quiet is it, do you still need ear pro?No, it states Carbine length gas system. The Gas Block adjustment has been screwed in, but gets tight or is hitting on the gas tube. Something is preventing it from screwing in far enough to cut the gas flow. Just haven't had time to look at it yet. My chart shows min. port size (for 11.5" barrel) as .086" and .094" as max. Even though my barrel is 12.5" long this range should still work fine (14.5" barrel uses a smaller port size; .063" - .078"). I just picked something in the middle of the 11.5" barrel port size range (.090"). Port size was not as critical since I planned on using an adjustable gas block. The only reason I believe I could get by with the .086" gas port is that it just feels as if the bolt is blowing back a little hard. That is completely subjective, with absolutely no basis in anything scientific or measurable. I just believe it would cycle (using 7.62 Surplus ammo) with the minimum .086" port size. Suppressor is a YHM Q.D. Phantom. Suppressor works fine, rifle cycles every time, and no hearing protection is required. Sounds a lot like disconnecting a large air hose. It could be shot indoors without hearing protection (even though it would be louder). Because of the YHM weight (28.6 oz.) I want to run a Titanium .308 can (13 oz.). Hope to pick one up in the near future. I do not build rifles for a living and all of this is the things that I thought would work. It, just as easily, could have gone the other direction. I think once the bolt gets worked in it will be fine. 65 rounds through it a couple weekends ago. Plan to run another 65 - 100 rounds through it tomorrow. I figured 300 rounds or so should wear the bolt/upper receiver in and hopefully keep from having to spray (lube) the bolt so much. I am very pleased with the outcome and the rifle is a blast to shoot. Unsuppressed it is loud, suppressed it is reasonably quiet and could be shot all day without hearing protection. Id post a pic, but have not had time to read the instructions. Maybe I can post a pic later. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 26, 2011 Report Share Posted November 26, 2011 Very impressive guesstimating on all your design specs. Having never done anything with SBR's or suppressed, maybe I can ask you, I've heard that the suppressed guns really get dirty fast when shooting, is that why you need to keep lubing it? Also, when you said that you turned/ported the barrel for carbine length, I'm assuming your referring to AR 15 carbine length, (around 7 1/2", is that correct? And finally, you mentioned that the bolt cycling feels subjectively harder than you had expected and that you could have gone with a smaller barrel port size, do you think that a heavier buffer might soften the bolt cycling impulse down a little? Quote Link to comment Share on other sites More sharing options...
jlbell Posted November 26, 2011 Author Report Share Posted November 26, 2011 Very impressive guesstimating on all your design specs. Having never done anything with SBR's or suppressed, maybe I can ask you, I've heard that the suppressed guns really get dirty fast when shooting, is that why you need to keep lubing it? Also, when you said that you turned/ported the barrel for carbine length, I'm assuming your referring to AR 15 carbine length, (around 7 1/2", is that correct? And finally, you mentioned that the bolt cycling feels subjectively harder than you had expected and that you could have gone with a smaller barrel port size, do you think that a heavier buffer might soften the bolt cycling impulse down a little?Running a suppressor, you get a lot of blowback! It holds a lot of gas/powder, etc. in and back pressure sends it back up the barrel. I always run the PRi Gas Buster charging handle which redirects the blowback out of your face. So, yes, the gun runs dirtier. But no, I don't think that is the problem with having to lube. It's just that all parts are brand new and it needs to wear in. Shot another 65 rounds Friday, all suppressed, without any problems. Heavier buffer probably would soften the bolt recoil, but after this Friday, it doesn't seem to be a real problem. And yes Carbine length gas system is just the standard AR15 carbine length (approx. 7 1/2"). Rifle worked well this week and is a lot of fun to shoot. Weight is good (unsuppressed), recoil is mild, accuracy seems good (bowling pins at 50 yds.), with red dot. Right now the only negative, for me, is that the YHM Steel Suppressor is a little heavy at the end of the barrel. A Titanium Suppressor will lighten it up right at a pound, which will really make a difference. Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 27, 2011 Report Share Posted November 27, 2011 Thanks for the info, I'm looking to seeing some pictures of it! Quote Link to comment Share on other sites More sharing options...
jlbell Posted November 27, 2011 Author Report Share Posted November 27, 2011 Jgun,send me your email address and I'll email you a couple pics of the rifle.jerrybell@mindspring.com Quote Link to comment Share on other sites More sharing options...
Jgun Posted November 27, 2011 Report Share Posted November 27, 2011 jlbell,Sent you an email. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.